Will an Emerald Crab eat my corals?

They are overwhelmingly reef-safe and typically ignore corals. However, if they are starving and there is no algae or detritus left in the tank, they may resort to picking at fleshy soft corals or zoanthids.

Do they really eat bubble algae?

Yes, Emerald Crabs are one of the few marine animals that consistently eat Valonia (bubble algae). While individual preferences can occasionally vary, they are widely considered the best natural control method available.

Why does my crab look dead but completely hollow?

Your crab has likely moulted! As they grow, they shed their old exoskeleton in one piece. The moult looks exactly like a dead crab, but the actual, slightly larger crab will be hiding nearby while its new shell hardens.

Can I keep more than one Emerald Crab together?

Yes, provided the tank is large enough and has sufficient live rock. In smaller nano tanks, it is best to keep just one, as males can become territorial if forced into close quarters.

Do I need to feed my Emerald Crab?

If your tank is very clean and lacks algae, absolutely. You should offer them dried seaweed (nori) and sinking pellets to ensure they don't starve or turn to nipping corals.

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Emerald Crab

Mithraculus sculptus

The Emerald Crab is a highly popular, reef-safe scavenger prized by aquarists for its remarkable appetite for nuisance bubble algae.

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Ideal Conditions

Temperature: 23 – 26 °C
pH range: 8.1 – 8.4
Hardness: 8 – 12 dGH
Salinity: 1.023 – 1.025 SG
Adult size: 3 – 5 cm
Nano Reef: 30 litres+
Lifespan: 2 – 4 years
Diet: Omnivore
Water flow: moderate
Preferred setup: Ample live rock with crevices and hiding spaces.

About the Emerald Crab

Hailing from the warm, shallow reefs of the Caribbean and the Gulf of Mexico, the Emerald Crab (*Mithraculus sculptus*) is a cornerstone of the marine cleanup crew. Characterised by its flattened, highly textured green carapace and spoon-tipped claws, it is perfectly adapted to scraping algae from porous rock. They are predominantly nocturnal or crepuscular, though they often become bold enough to forage during the day in established setups. Highly valued for their utility, they are a practical and fascinating addition to both sprawling reef systems and compact nano aquariums.

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Is it right for your tank?

Check Emerald Crab against a habitat you already keep. We compare water type, temperature, pH, adult size, group needs and everything living there.

Where Emerald Crab comes from

Native range

Caribbean Sea, Gulf of Mexico, Western Atlantic

Countries / regions

United States, Bahamas, Cuba, Mexico

Habitat

Shallow coastal reefs, rubble zones, and seagrass beds with abundant rocky hiding places.

Commonly collected among live rock and shallow coastal reefs throughout the Caribbean basin.

Similar species

Sally Lightfoot Crab
Percnon gibbesi
Ruby Mithrax Crab
Mithraculus forcep
Porcelain Crab
Neopetrolisthes maculatus

We’re adding more full species guides as the catalogue grows.

Compatible tank mates

Reviewed guidance based on this species' recorded traits.

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Ocellaris Clownfish
Amphiprion ocellaris
Occupy different zones of the aquarium and will completely ignore each other.
Great
Peppermint Shrimp
Lysmata wurdemanni
Peaceful co-inhabitants that tackle different nuisance pests (Aiptasia vs bubble algae) as part of a robust cleanup crew.
Great
Neon Goby
Elacatinus oceanops
While usually perfectly safe, very large or starving crabs might opportunistically grab at small, bottom-dwelling fish.
Compatible with caution
Picasso Triggerfish
Rhinecanthus aculeatus
Triggerfish are specialised invertebrate predators that will actively hunt, crush, and eat Emerald Crabs.
Not recommended
Longnose Hawkfish
Oxycirrhites typus
Hawkfish are ambush predators that frequently target crustaceans and will eagerly eat small crabs.
Not recommended

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